Beta-Adrenergic receptors are present in a number of cultured cell lines and their ability to function is altered by a number of factors in the culture medium and the length of time in culture. When astrocytoma cells (C6 glioma) undergo many generations, the ability of the cell to respond to catecholamines is profoundly reduced. This reduction in responsiveness is due to several factors including a decreased number of Beta-adrenergic receptors and increased phosphodiesterase activity; this results in a 5-fold decrease in cAMP half-life in the cell. Ultimately, these findings may be important in understanding age-related phenomena. When either old or young astrocytoma cells are exposed to catecholamines, a rapid and specific desensitization to a second stimulation by catecholamines occurs. This seems to occur as a two-step process. The first step involves uncoupling of Beta-adrenergic receptor form adenylate cyclase; the second step involves internalization and loss of receptor molecules.